In UMTS LTE the downlink control channel PDCCH (Physical Downlink Control Channel) carries information such as resource allocation for uplink or downlink transmission. A PDCCH message can use 1, 2, 4 or 8 Channel Control Elements (CCEs or resource elements)—referred to as CCE aggregation levels 1, 2, 4 or 8. PDCCH messages can be transmitted using one of a set of available message formats (e.g. with different signal characteristics such as number of information bits and channel coding rate, The different formats are referred to in the LTE specifications as “DCI formats”. In addition, different destinations or purposes for the PDCCH messages may be indicated by different scrambling sequences applied to the message CRC (in the LTE specifications the different scrambling sequences correspond to different identities, referred to as RNTIs). Different RNTIs are used to distinguish UE-specific PDCCH messages intended for a particular UE, from Common PDCCH messages intended for reception by more than one UE. In the case of a PDCCH message intended for reception by more than one UE, this could be for a defined group of UEs, or any UE.
A mobile station, like a UE in LTE, does not know in advance the location in CCE space of messages intended for it. In principle, the mobile station could attempt to blindly decode all the possible PDCCHs with different starting positions in the CCE space and thus receive any messages intended for that mobile station. However, if the CCE space is large the processing complexity is prohibitive. Therefore a more limited search is configured which consists of a number of search spaces. A UE may search a given search space for messages with one of more DCI formats. For simplicity of explanation, we may consider only one DCI format and one RNTI in a UE-specific search space, but the same discussion is applicable for multiple DCI formats, multiple RNTIs and for UE-specific and Common search spaces.
A search space is a set of aggregated CCEs (with a certain aggregation level) within which a mobile station (or user equipment (UE) or secondary station) performs blind decoding of all PDCCH payloads (DCI formats and RNTIs) that it assumes may be transmitted for that aggregation level. The set of PDCCH payloads that the UE assumes may be transmitted may be a subset of all possible PDCCH payloads defined by the LTE specification. Search spaces are defined per aggregation level; a secondary station thus can have up to four search spaces. For example, the search space of a UE for aggregation level 1 (referred to as 1-CCE) could consist of the CCEs indexed 3,4,5,6,7,8, while its search space for aggregation level 8 could consist of the two resource sets of aggregated CCEs consisting of the CCEs indexed by 1,2, . . . 8 and 9,10, . . . , 16, respectively. In this example, the UE thus performs six blind decodings for 1-CCEs and two blind decodings for 8-CCEs.
The LTE specification currently requires the UE to perform the following in the search spaces designed for UE-specific PDCCH messages on a single carrier (i.e. UE-specific search space (UESSS)):
6 decoding attempts of 1-CCE aggregation
6 decoding attempts of 2-CCE aggregation
2 decoding attempts of 4-CCE aggregation
2 decoding attempts of 8-CCE aggregation
In addition the UE is required to perform the following in a search space designed for Common PDCCH messages on a single carrier (i.e. Common search space):
4 decoding attempts of 4-CCE aggregation
2 decoding attempts of 8-CCE aggregation
In a conventional wireless system represented on FIG. 1, a primary station 100 exchanges data with a plurality of secondary stations 110. To transmit its data, the primary station 100 transmits sends its data on a downlink data channel 101. This downlink data channel may be adjusted over time in response to several criteria, e.g. Quality of Service, interference, downlink channel quality. To inform the secondary stations 110 of these changes, the primary station 100 transmits control data (or signalling) to the secondary stations 110 on a downlink control channel 102. Similarly, the secondary stations 110 transmit their data on an uplink control channel 111. Moreover, an uplink control channel 112 is used by the secondary station to request resources for transmissions and/or for providing the primary station with feedback on the downlink transmissions or on the channel quality state.
In many wireless systems, like mobile communication systems as UMTS LTE (Long Term Evolution), or LTE Advanced, signalling on control channels 102 or 112 is provided so that it indicates the particular time-frequency transmission resources to which the data is mapped on the data channels 101 or 111, and the transmission scheme used for that data (i.e. the format/mode in which the data itself is transmitted). In addition the secondary station may provide channel state feedback intended to assist the primary station in scheduling transmission to suitable secondary stations using appropriate transmission resources and transmission scheme. Therefore, in general a transmission mode may be defined by one or both of a transmission scheme used by the primary station or the type of feedback provided by the secondary station.
For UMTS LTE downlink communication, the relevant downlink control channel 101 that includes resource allocation and transmission format information is known as Physical Downlink Control Channel (PDCCH). The message a PDCCH carries is known as Downlink Control Channel Information (DCI). The resource allocations for different transmission modes are typically indicated using different DCI formats. The secondary station (here a User Equipment or UE) is configured to receive a limited number of different DCI formats from the possible set. Therefore configuring the types of DCI format expected by the UE in the PDCCH directly controls the transmission modes which may be expected by the secondary station, and a particular transmission mode is signalled by the particular DCI format used for resource allocation. The DCI Format contents can also be associated with a transmission mode (e.g. in LTE a 1 bit field indicated uplink or downlink resources).
The following PDCCH properties may be configured/reconfigured at the secondary station by higher layer signalling from the primary station:                DCI formats (depending on transmission mode)        DCI format size (e.g. addition/removal of CIF field)        PDCCH Search spaces        Carriers (additional CCs for carrier aggregation, anchor carrier, or handover).        
However, because of the delays inherent in higher layer (RRC) signalling the primary station does not know exactly when the secondary station applies the new configuration. Therefore, there is a possibility that a PDCCH message (e.g. with a particular new DCI format) may be transmitted by the eNB, but not received by the UE (either because it has not yet applied the reconfiguration). Similarly PDCCH messages may be lost if the new configuration is applied earlier than expected by the eNB.